1. Field of the Invention
This invention applies to fiber optic networks that use a combination of time and wavelength division multiplexing and transport communication protocols that require either loss of light (LOL), open fiber control (OFC), or a combination of the two states to be transported across the network.
2. Description of the Related Art
Recent advances in fiber optic dense wavelength division multiplexing (DWDM) equipment have made more efficient use of the fiber's available bandwidth by using a combination of wavelength division multiplexing (WDM) and time division multiplexing (TDM). Typically, in a TDM/WDM system, multiple input signals with data rates up to 1-2 gigabits/second (Gbit/s) are time multiplexed into a single, high-speed data stream. This is then modulated onto one of the optical wavelengths in a wavelength division multiplexing (WDM) network, which may be operating at 10 Gbit/s or faster. This approach provides a cost-effective way to scale the capacity of an optical network and is currently being applied to industry-standard protocols such as Gigabit Ethernet, Fibre Channel, Asynchronous Transfer Mode (ATM), and others.
Some data communication protocols require special accommodations to operate in this environment. For example, IBM has developed a set of protocols known as Inter-System Channel (ISC) links, which are used for clustering of mainframe computers in a Geographically Dispersed Parallel Sysplex (GDPS) architecture. (Geographically Dispersed Parallel Sysplex and GDPS are trademarks of IBM Corporation.) This approach is used for high availability and disaster recovery at larger companies worldwide and requires the extension of ISC links over DWDM networks to distances of 50-100 kilometers (km) or more. Until recently, there was no need to time multiplex the ISC channels, as the maximum data rate per wavelength in a WDM network was about 2.5 Gbit/s, approximately the same as an ISC channel. (These channels can operate in either peer mode at 2.125 Gbit/s or compatibility mode at 1.0625 Gbit/s; the compatibility mode links also use a version of open fiber control (OFC) protocols.) With the recent increase in WDM per wavelength data rates to 10 Gbit/s and beyond, it is necessary to find a method for time multiplexing several ISC channels over a common wavelength so that the GDPS architecture remains cost competitive.
This operation requires two essential steps. The first is a method for speed matching the FIFO buffers between the ISC channel and the WDM network. This requires knowledge of IBM data frame structures, and algorithms to accomplish this are described in U.S. Patent Application Publication 2005/0100337 (DeCusatis et al.), incorporated herein by reference. The second essential step is to accommodate time multiplexing of the channel initialization and control information, including open fiber control (OFC) and loss of light (LOL) propagation.
OFC is described in U.S. Pat. Nos. 6,356,367, 6,359,709 and 6,359,713 (DeCusatis et al.), as well as in U.S. Pat. No. 6,438,285 (DeCusatis et al) and U.S. Patent Application Publication 2003/0072516 (DeCusatis et al.), all of which are incorporated herein by reference. As explained in the referenced patent application publication, OFC is a laser eye safety interlock implemented in the transceiver hardware; a pair of transceivers connected by a point-to-point link must perform a handshake sequence in order to initialize the link before data transmission occurs. Only after this handshake is complete will the lasers turn on at full optical power. If the link is opened for any reason (such as a broken fiber or unplugged connector), the link detects this and automatically deactivates the lasers on both ends to prevent exposure to hazardous optical power levels. When the link is closed again, the hardware automatically detects this condition and reestablishes the link. OFC is defined for various laser wavelengths and data rates in the ANSI Fibre Channel Standard; the OFC timing and state machine are also defined in this standard. OFC is still required to interoperate with other devices attached to the fiber links, even where it no longer serves a laser safety function.
LOL is described in the above-identified U.S. Pat. Nos. 6,356,367, 6,359,709 and 6,359,713 and U.S. Patent Application Publication 2003/0072516, as well as in such patents as U.S. Pat. No. 5,504,611 (Carbone et al.) and U.S. Pat. No. 5,136,410 (Heiling et al.), incorporated herein by reference. Even links that do not implement OFC protocols must sometimes propagate a loss of light (LOL) condition along the length of the fiber. As explained in the referenced patent application publication 2003/0072516, propagating loss of light is not the same as sending a long string of zero data on the link; the attached computer equipment must be able to determine the difference between an open optical connection and a long run of zeros (potentially corrupted data) since the error recovery is different in each case.
The present invention addresses these two critical link states, LOL and OFC, which must be propagated through a TDM/WDM network to insure proper functionality of an ISC channel.
Previously, this control information was passed across WDM networks on a per-wavelength optical supervisory channel (OSC). This is illustrated in FIG. 1, which shows a prior art system 100 containing a WDM transmitting node 102 and a WDM receiving node 104 coupled via a network 106. (This example has been simplified somewhat, since each of the node 102 and 104 contains both transmitting and receiving functions.)
Transmitting node 102 contains a plurality of input channels, each of which drives a common WDM multiplexer 120. In each of these input channels, an optical signal 108 on a link from a client (not shown) drives an optical-to-electrical (OE) transducer or optical receiver (RX) 110 to produce an electrical output signal 112. This electrical signal 112 is combined with an electrical overhead control signal 114 and the result fed to an electrical-to-optical (EO) transducer or optical transmitter (TX) 116. Transducer 116 has an internal laser (not separately shown) that provides an optical signal 118 of a particular wavelength to WDM multiplexer 120. WDM multiplexer 120 combines the optical signals 118 from all of these input channels (which have different wavelengths) to provide a single multiple-wavelength optical output signal 122 to the network 106.
Correspondingly, at the receiving node 104, a WDM demultiplexer 126 takes a multiple-wavelength optical input signal 124 from the network 106 and separates it into multiple optical signals 128 of different wavelengths that are processed in respective output channels. In each of these output channels, an optical receiver 130 converts the optical signal 128 to an electrical signal 132, from which an overhead control signal 134 is extracted using well-known techniques. Finally, an optical transmitter 136 takes the electrical signal 132 from which the control signal was 134 extracted and, using another internal laser, converts it to an optical output signal 138 corresponding to the original input signal 108.
In the system 100 illustrated in FIG. 1, an input optical data stream 108 is converted into electrical form 112, then remodulated onto another laser signal 118 whose wavelength is compatible with the WDM network 106. In the process, overhead bits 114 that carry network management information for this wavelength are added to the data flow. This overhead channel does not occupy a significant fraction of the available bandwidth (perhaps a few percent), and is confined within the WDM network 106; it is stripped off by the receiver function at the destination WDM node 104. In this manner, if there is a fiber or component failure anywhere in the link, the WDM equipment can deactivate both its network laser connection and client laser connection. Similarly, if the link is equipped with OFC protocols, the entire optical link can be deactivated until the failure is corrected; at that time, OFC automatically reinitializes the end-to-end link. Otherwise, the WDM interface transparently passes along any input data to the output node.
This approach cannot be used, however, if one plans to time multiplex several channels of ISC traffic. In this latter scenario, let us consider, for example, LOL propagation. If there is an equipment failure in the TDM stage that affects only one ISC channel, it is no longer possible to disable the lasers throughout the link, since they are still carrying ISC traffic for other input channels. It is also not possible to simply transmit all zeros. This is true for many reasons, including the fact that such a transmission would violate disparity on the ISC link. It would be misinterpreted as a data error and inhibit proper channel error recovery, and the clock recovery circuits in the receiver would drift out of lock under these conditions. Similar considerations apply to OFC propagation when time multiplexing several channels of ISC traffic.
Previously, there have been various efforts to multiplex various communication protocols in optical networks. The following patents are representative.
U.S. Pat. No. 6,587,615 (Paiam) describes an optical wavelength demultiplexer with a substantially flat output response within its passband. This is accomplished by using a two-stage optical wavelength multiplexing process, in which the first WDM has a free spectral range approximately equal to the second WDM. Various embodiments are described, including resonant optical cavities, array waveguide gratings, and others. This patent only addresses the optical spectral properties of a WDM system. It does not incorporate time division multiplexing technology and does not address LOL or OFC state propagation across a network.
U.S. Pat. No. 5,814,557 (Otsuka et al.) describes a method and apparatus for scrambling the polarization of optical signals in a WDM system to suppress nonlinear effects and improve transmission fidelity. Various embodiments are proposed, including per wavelength polarization scramblers and a two-stage wavelength combination scheme with a scrambling stage in between. This patent only addresses the nonlinear effects in a long-haul WDM system that can arise from variations in the optical polarization. It does not incorporate time division multiplexing technology and does not address LOL or OFC state propagation across a network.
U.S. Patent Publication 2003/0081294 (Lee et al.) describes a free-space WDM system which couples the received channels into an optical fiber to facilitate the use of optical amplifiers. A light beam emitting and focusing unit is described to facilitate this coupling, which includes an optical circulator, WDM coupler, and amplified spontaneous emission fibers. This patent does not incorporate time division multiplexing technology and does not address LOL or OFC state propagation across a network.
The above-mentioned family of U.S. Pat. Nos. 6,359,709, 6,359,713 and 6,356,367 describe a method, apparatus, and computer program product for a fiber optic network that allow OFC conditions to propagate across a WDM network. This is accomplished by using an outband signal which carries the OFC state; an alternative embodiment using an electrical wrap mode is also presented. The technology described in these patents only applies to a WDM system, without TDM, and is not extendable to include TDM systems. In fact, the approach described in these patents will not work in a TDM environment. Thus, the patents describe using a per wavelength control channel to propagate OFC state information, which means that only one data channel per wavelength can be supported. These patents also do not address LOL propagation across a WDM or TDM network.